U.S. patent application number 11/592160 was filed with the patent office on 2008-05-08 for fuel cell.
This patent application is currently assigned to TOAGOSEI CO., LTD.. Invention is credited to Hideki Hiraoka, Yoshinori Kimata, Yoshinori Yamada.
Application Number | 20080107926 11/592160 |
Document ID | / |
Family ID | 39360071 |
Filed Date | 2008-05-08 |
United States Patent
Application |
20080107926 |
Kind Code |
A1 |
Yamada; Yoshinori ; et
al. |
May 8, 2008 |
Fuel cell
Abstract
A fuel cell is provided in which a unit comprising a unit cell
and a current collector or a stack comprising a plurality of unit
cells and current collectors is supported by an end plate, wherein
the end plate employing a printed board having printed wiring on
its surface.
Inventors: |
Yamada; Yoshinori; (Aichi,
JP) ; Hiraoka; Hideki; (Aichi, JP) ; Kimata;
Yoshinori; (Aichi, JP) |
Correspondence
Address: |
KRATZ, QUINTOS & HANSON, LLP
1420 K Street, N.W., Suite 400
WASHINGTON
DC
20005
US
|
Assignee: |
TOAGOSEI CO., LTD.
Tokyo
JP
|
Family ID: |
39360071 |
Appl. No.: |
11/592160 |
Filed: |
November 3, 2006 |
Current U.S.
Class: |
429/469 ;
429/522; 429/534 |
Current CPC
Class: |
H01M 8/0269 20130101;
H01M 8/0247 20130101; Y02E 60/50 20130101; H01M 8/0232
20130101 |
Class at
Publication: |
429/12 |
International
Class: |
H01M 8/02 20060101
H01M008/02 |
Claims
1. A fuel cell in which a unit comprising a unit cell and a current
collector or a stack comprising a plurality of unit cells and
current collectors is supported by an end plate, the end plate
employing a printed board having printed wiring on its surface.
2. The fuel cell according to claim 1, wherein the current
collector is a metal mesh having a surface roughness, as an
arithmetic mean roughness, of a surface in contact with a catalyst
layer or a diffusion layer in the unit cell of no greater than 10
.mu.m.
3. The fuel cell according to claim 1, wherein the current
collector is a metal mesh having a surface roughness, as an
arithmetic mean roughness, of a surface in contact with a catalyst
layer or a diffusion layer in the unit cell of no greater than 1
.mu.m.
4. The fuel cell according to claim 1, wherein the current
collector is a metal mesh.
5. The fuel cell according to claim 1, wherein the current
collector is a perforated metal, an expanded metal, or a wire
netting.
6. The fuel cell according to claim 1, wherein the current
collector is an expanded metal.
7. The fuel cell according to claim 4, wherein a material for the
metal mesh is gold, platinum, or titanium.
8. The fuel cell according to claim 4, wherein a material for the
metal mesh is titanium.
9. The fuel cell according to claim 1, wherein the printed board
has a thickness of 2 mm or less.
10. The fuel cell according to claim 1, wherein the printed board
is a printed board using a glass epoxy board, a glass composite
board, a paper epoxy board, a paper phenol board, a polyimide
board, a PEEK (registered trademark) resin board, or a printed
wiring board with a metal core.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The present invention relates to a fuel cell structure.
[0003] 2. Description of the Related Art
[0004] A fuel cell is a device that generates an electromotive
force by employing a gaseous or liquid fuel as a fuel and supplying
oxygen or air as an oxidizing agent; it normally has a structure in
which a proton-conducting electrolyte is sandwiched by electrodes
containing an oxidation catalyst, and can give a desired
electromotive force. The application of such a fuel cell to an
electric automobile or stationary power generation has been
expected; the development of practical applications is progressing
and, in addition to these uses, the application to new uses is
being investigated, utilizing the advantage that a reduction in
weight and size is easy. For example, there is the use as a new
power source in the replacement of existing dry batteries or
rechargeable batteries in portable electrical appliances.
[0005] With regard to a small, lightweight fuel cell that can be
used in a portable electric appliance, research has been carried
out into various aspects, and selection is generally made from a
system called a PEMFC, which employs hydrogen as a fuel, and a
system called a DMFC, which employs a liquid fuel such as methanol.
In a PEMFC, a mechanism for storing or generating hydrogen gas is
essential, and there is the problem that the volume, weight, and
cost of this mechanism are large. On the other hand, in a DMFC,
there is no need to store or generate hydrogen gas, but since a
catalytic oxidation reaction of a liquid fuel is slow, the output
density is low, which is a problem, and it can be said to have
merits and demerits. Although the present invention may be applied
to either of these two systems, its desirable effects are
particularly outstanding when it is used in a DMFC.
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0006] Since the theoretical power generation voltage of a fuel
cell is 1.23 V whereas the working voltage of a general dry battery
is 1.5 V, when a fuel cell is used as a replacement for a dry
battery, it is necessary to connect a plurality of unit cells in
series. In general, they are often used in a stack structure in
which a separator is interposed between a plurality of unit cells,
the separator being electrically conductive and allowing a fuel or
gas to diffuse therethrough. The separator is made of an
electrically conductive material such as a corrosion-resistant
metal molding or a solid carbon block, and plays a role as a
current collector that conducts the current that is generated by
making intimate contact with an electrode of the fuel cell and that
connects to external wiring, and a role of supplying a fuel or an
oxidizing agent to a catalyst electrode by means of a flow path or
diffusion employing a channel, etc. provided on the surface. The
separator is generally required to have very high quality in terms
of high precision, high rigidity, high corrosion resistance, low
electrical resistance, etc., and there is therefore the problem
that it is thick and heavy and has a high cost of as much as 1/3 of
the production cost of a fuel cell main body.
[0007] As such a separator, one that is formed by subjecting a 4 mm
thick stainless steel plate to a surface treatment is disclosed in,
for example, JP-A-2001-250565 (Patent Publication 1) (JP-A denotes
a Japanese unexamined patent application publication), but as
described above it is thick and heavy and has a high cost.
(Patent Publication 1) JP-A-2001-250565
[0008] On the other hand, end plates are disposed at opposite ends
of a unit cell, or at opposite ends of the stack structure, in
which the separator is interposed between a plurality of unit
cells, and in order to apply a strong compression force evenly to
the structure, the end plate is normally a plate having a large
thickness and very high rigidity such as a thick special metal
plate or an engineering plastic; machining is difficult, the cost
of the material is high, and both the volume and weight are large,
which are the main causes of the increase in overall cost and
volume of the fuel cell.
[0009] The present invention has been proposed taking into account
such conventional circumstances, and it is an object thereof to
provide a small, lightweight, and inexpensive fuel cell, the fuel
cell being capable of being connected in series without a high cost
separator or end plate.
BRIEF SUMMARY OF THE INVENTION
[0010] In order to achieve the above-mentioned object, the fuel
cell of the present invention employs a printed board as an end
plate, a fuel cell main body can be integrated with a peripheral
circuit such as a terminal for electrical connection to an external
circuit or wiring when connecting a plurality of unit cells in
series, and the mechanical strength of the fuel cell main body and
good contact with an electrode can be maintained by means of an
inexpensive general-purpose material.
[0011] Furthermore, the present invention limits the surface
roughness of a metal mesh to a specific value, thus guaranteeing
good contact conductivity with a fuel cell electrode even under a
relatively low pressure. As a result, a small, lightweight, and
inexpensive fuel cell that gives a high output can be realized.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] (FIG. 1) A sectional view illustrating one example of the
fuel cell of the present invention.
[0013] (FIG. 2) A front view of MEAs, end plates, and a fuel tank
of a fuel cell having the structure shown in FIG. 1.
[0014] (FIG. 3) A front view of an end plate of the fuel cell with
the structure shown in FIG. 1 on the surface of which a printed
wiring pattern (areas shown in white in the peripheral region) is
formed.
EXPLANATION OF REFERENCE NUMERALS
1 Current collector
2 Securing screw
3 End plate
4 Gasket
5 End plate having printed wiring
6 MEA
7 Fuel tank frame
8 In-tank spacer
9 Wiring pattern
BEST MODE FOR CARRYING OUT THE INVENTION
[0015] The fuel cell of the present invention is explained in
detail below by reference to drawings.
[0016] FIG. 1 illustrates one example of the fuel cell of the
present invention. In this example, two unit cells are disposed so
as to face each other with the cathode side on the outside and a
common fuel tank in the middle, and one end plate is a printed
board having printed wiring on the surface. With regard to an MEA
(electrolyte membrane equipped with a catalyst layer), which is
inserted between the end plate at either end and the fuel tank in
the middle, current collectors are in intimate contact with the
cathode side and the anode side thereof, and end parts of the
current collectors are connected to a circuit on the end plate.
[0017] FIG. 1 illustrates a combination of two unit cells, but it
is also possible to form a cell from only one unit cell; in this
case members on the left-hand side relative to the fuel tank (the
MEA and the end plate on the left-hand side) are not needed, and
instead a wall that prevents fuel within the fuel tank from leaking
is provided on the left-hand side of the fuel tank. It is of course
possible to form a stack by arranging the two unit cells in
parallel instead of back-to-back or by putting three or more unit
cells into intimate contact, and in such cases since, unlike the
structure of FIG. 1, the cathode has a face that is not open to the
outside air, it is preferable to provide the cathode with a flow
path or a gap for supplying an oxidizing material such as air or
oxygen.
[0018] With regard to a method for supplying a fuel and an
oxidizing agent to the MEA, a system generally called a `passive
type` in which fuel and air are supplied by natural convection
gives the lightest weight and the lowest cost, and this system is
preferable for a small, lightweight fuel cell with relatively low
power. On the other hand, it is also possible to employ a system
called an `active type` in which one or both of a fuel and an
oxidizing agent are made to circulate by virtue of mechanical force
or pressure. In this case, extra cost and energy for equipment for
generating the mechanical force are needed and the fuel cell
becomes large, but it is unnecessary to make the cathode open to
the outside air, and a large amount of power can be taken out by
superimposing a large number of cell units in intimate contact. The
fuel cell of the present invention may suitably employ either of
these systems, but it is preferable to employ the passive type
since the features of small size, light weight, and low cost can be
exploited.
[0019] Each of the two unit cells shown in FIG. 1 has one MEA
incorporated thereinto. In this process, in order to prevent the
fuel and oxidizing agent from leaking, it is possible to interpose
a gasket or an O-ring between the two unit cells, provide a step in
the fuel tank and the end plate, or apply a sealant.
[0020] FIG. 2 illustrates a structure in which a total of eight
screw holes are opened in each member and the entirety is tightened
by a fastening material such as a screw. As shown in FIG. 1, the
screw runs through the entirety so as to fasten the end plates at
opposite ends, and fastening one end of a preferred current
collector to the end plate surface by means of the screw enables
reliable contact of the current collector with printed wiring on
the end plate surface. Furthermore, as shown in FIG. 2, by
fastening given current collectors of two, that is, front and back
cells, by means of the same screw the two cells can be connected in
series or in parallel. That is, in some cases the screw has a
function of fastening the plurality of unit cells or the stack as
well as a function of being a part of the circuit. The screw may be
made of a normal metal, or of a resin such as an engineering
plastic or nylon if insulation is required.
[0021] It is also possible to use an adhesive instead of a screw
for fastening. Although the adhesive does not have a function as a
part of the circuit, or provide contact between the current
collector and the printed wiring, when an appropriate adhesive is
selected, utilizing its function of preventing leakage
advantageously gives a small and lightweight cell. Although the
adhesive is not particularly limited, it is preferable to use an
epoxy- or polyester-based adhesive since it has high adhesion and
chemical durability and it is stable for a long period of time even
in contact with a fuel or an oxidizing agent.
[0022] A fuel tank frame shown in FIG. 1 is a member for retaining
a fuel or making it flow, and a material that does not interfere
with the fuel and is electrically insulating may be used. The frame
and a gasket may be integral or separate. It is preferable to
insert an in-tank spacer into the interior of the frame in order to
oppose a pressure applied from the end plate to the MEA. The
in-tank spacer may be one that is resistant to fuel, and may employ
a polyethylene net, a plastic block into which a channel for fuel
to flow is engraved, etc. The in-tank spacer may be integral with
or separate from either the tank frame or the gasket, etc.
[0023] The MEA is formed from an electrolyte membrane with a
catalyst layer and a diffusion layer on both sides thereof. The
electrolyte membrane used may be one with proton conductivity; it
is particularly preferable to use a perfluoroolefin-based
ion-exchange membrane, represented by a Nafion (registered
trademark) membrane manufactured by DuPont, or an ion-exchange
membrane of a system in which pores of or a porous substrate are
filled with a polymer with ion-exchangeability, and it is also
preferable to use other hydrocarbon-based ion-exchange membranes or
inorganic ion-exchange membranes, etc. The catalyst layer and the
diffusion layer have generally known compositions and may be formed
by standard methods.
[0024] With regard to a fuel cell having the above-mentioned
configuration, the present invention is characterized by an end
plate and a current collector. With regard to the end plate, as
described above, a plate having a large thickness and very high
rigidity such as a carbon block, a thick special metal plate and,
in some cases, a polycarbonate or engineering plastic plate is
conventionally used; machining is difficult, the cost of the
material is high, and both the volume and weight are large, which
are the main causes of the increase in overall cost and volume of
the fuel cell.
[0025] In the present invention, a printed board is used as the end
plate, printed wiring is formed on at least one end plate, light
weight and low cost can thus be achieved, and by incorporating a
peripheral circuit of the fuel cell into the cell main body, the
reliability and the volume efficiency are improved. With regard to
a printed board that can be used in the present invention, in
addition to general-purpose standard products such as FR-4 (glass
epoxy board), CEM-3 (glass composite board), FR-3 (paper epoxy
board), and FR-1 (paper phenol board), a polyimide board, a PEEK
(registered trademark) resin board, and a printed wiring board with
a metal core, etc. may preferably be used in a similar manner.
[0026] As a method for forming printed wiring, a conventional
patterning method for a metal-clad laminate such as a copper-clad
laminate, etc. may preferably be used. Copper, aluminum, silver,
gold, nickel, titanium, etc. metal wiring is formed on the surface
of the board. An electronic component such as an LED or a resistor
is mounted on the printed wiring formed on the end plate so as to
display the amount of fuel remaining or voltage and current values,
and it is also possible to form a converter for increasing the
voltage, a stabilized power supply circuit for controlling the
output, an auxiliary circuit for controlling supply of a fuel or an
oxidizing agent, etc. Conventionally, these components are usually
disposed at a different location from that for the fuel cell main
body and connected via a lead, and the present invention therefore
enables the number of components to be greatly decreased and the
dimensions to be reduced. Furthermore, the integration and
reliability can be further improved by a method in which the board
is multilayered or resin-molded, etc.
[0027] As hereinbefore described, an end plate equipped with
printed wiring has high utility value, but there is the problem
that among the standard formats of printed board, which are used in
large amounts at low cost as general-purpose products, there is no
plate having a thickness of 4 to 100 mm, which has been used as an
end plate in the art. The reason why a thick plate is used as the
end plate is because tightening the entire fuel cell with a strong
force gives a uniform contact surface pressure to the MEA and the
separator or the current collector, thus reducing the contact
resistance between a carbon layer on the MEA surface and the
current collector.
[0028] Patent Publication 1 discloses a method in which, in order
to reduce the contact resistance, the surface of a separator is
coated with a metal having corrosion resistance, but its effect is
not sufficient. In an example of this publication, it is disclosed
that a stainless steel plate having a thickness of 4 mm is used and
gold plating is carried out.
[0029] Furthermore, although in this publication a corrosion
resistance effect is not demonstrated, there is the well-known
problem that, in accordance with a solid plating method described
in the above-mentioned method, pin holes remain and it is
impossible to prevent the base material from being partially
corroded, and this method cannot be said to be excellent in terms
of corrosion resistance.
[0030] The first aspect of the present invention employs a printed
board as an end plate; since a standard printed board format, which
is generally distributed at a low price, has a thickness of 2 mm or
less, and does not have the same rigidity as that of a stainless
steel plate having a thickness of 4 mm as disclosed in, for
example, Patent Publication 1, in order to achieve this rigidity it
is necessary to specially prepare a thick printed board or
superimpose a plurality of printed wiring boards.
[0031] However, by the use of a specific current collector in
accordance with the second aspect of the present invention, a
sufficiently low contact resistance can be realized by an end plate
comprising a printed board having a thickness of 2 mm or less.
[0032] A current collector of a conventional normal fuel cell is a
rigid separator with a flow path cut in the surface as disclosed
in, for example, Patent Publication 1 above, and it employs a
system in which it makes contact with an MEA through a face other
than the flow path, and a fuel or an oxidizing agent is made to
flow through the flow path.
[0033] On the other hand, the specific current collector referred
to in the present invention is a metal mesh, and since the aperture
area can be made large, sufficient flow of a fuel or an oxidizing
agent can be obtained by virtue of diffusion without specially
cutting a flow path.
[0034] Although a mesh current collector itself is already known,
since the contact resistance is high, the generated power cannot be
taken out efficiently unless the contact pressure with an MEA is
increased considerably, with the result that the current collector
cuts into the MEA, and a thick and highly rigid end plate is
therefore essential.
[0035] As a result of an intensive investigation by the present
inventors, it has been found out that, with regard to a mesh-like
current collector with a smooth surface that is in contact with an
MEA, the contact resistance is low without applying such a high
contact surface pressure. Specifically, a current collector is used
that has a mesh-like structure such as a punching metal, an
expanded metal, or a wire netting and whose roughness on the
surface that makes contact with the MEA is no greater than a
predetermined value. Among these mesh-like current collectors, the
punching metal has a limited aperture area and cannot have a very
fine pattern for the aperture and the contact part; there is thus
the problem that it is difficult to obtain a high output due to
restrictions in conductivity and diffusion in the lateral direction
on the surface of the MEA, but it may preferably be used by
carrying out improvements such as cutting a channel in the surface
or corrugating.
[0036] With regard to the wire netting, since it is woven, the
contact surface has severe irregularities; in order to obtain a
contact area a high contact pressure is required, and there is also
the problem that conductivity within the wire netting in a
direction perpendicular to the mesh pattern is poor, but it may
preferably be used by squashing or welding the weave.
[0037] Therefore, the best mesh-like current collector is an
expanded metal. As a material for these mesh-like current
collectors, those having good corrosion resistance and good contact
resistance with an MEA may be used. Gold and platinum, which have
low contact resistance, are preferably used; whereas titanium in
particular is corrosion resistant and has light weight and low
price, it has a large contact resistance, and particularly
remarkable effects can be obtained.
[0038] The specification of an expanded metal is defined in terms
of plate thickness, line width, aperture ratio, etc. It is
undesirable for the plate thickness to be too thin since the
strength is insufficient or for it to be too thick since this
results in heavy weight and high cost; the plate thickness is
preferably 0.01 to 1 mm, and more preferably 0.05 to 0.5 mm. With
regard to the line width, it is difficult to produce a very thin
one, and it is preferably 0.02 to 1 mm, and more preferably 0.05 to
0.5 mm. The aperture ratio is preferably at least 50% but no
greater than 95%, and more preferably at least 60% but no greater
than 90%. The above-mentioned definitions are usual for expanded
metal, and in addition thereto the present inventors have found
that the contact resistance depends on the surface roughness of the
contact area with the MEA, and by clarifying a preferred surface
roughness the present invention has been accomplished.
[0039] A surface roughness preferred in the present invention is no
greater than 10 .mu.m as an arithmetic mean roughness (Ra), and
more preferably no greater than 1 .mu.m. The arithmetic mean
roughness referred to here is defined in accordance with JIS B0901
(2001 edition), and is also defined as an arithmetic mean value of
the size of irregularities for a standard length set in accordance
with JIS B0633 (ibid). The surface of concern in the present
invention is a contact surface between the current collector and
the MEA, and the surface roughness, etc. on the side face of the
current collector that does not make contact with the MEA is of no
concern. The roughness of the actual surface may be measured by
means of an atomic force microscope, a laser microscope, a contact
type surface roughness meter, etc.
[0040] The reason why the contact resistance is influenced by the
surface roughness of the current collector, and why an optimum
value exists may be explained as follows.
[0041] When conduction is obtained by contacting the metal
separator or the current collector with the MEA, it is a well-known
fact that, if the contact surface pressure is less than a certain
value, the contact resistance becomes very high, and this is
clarified in Patent Publication 1 above. The reason therefor is
that since the outermost surface of a material such as a separator
or a current collector has micro irregularities that cannot be seen
by the eye, the true contact area is smaller than an apparent
contact area obtained by calculation. Furthermore, since carbon
paper, carbon cloth, carbon paste, etc., which forms the surface of
the MEA, has a very high roughness, the true contact area resulting
from simply making the two come into contact with each other
becomes very small, thereby increasing the contact resistance. It
is therefore usual for them to be squashed with a strong surface
pressure so as to increase the true contact area, but it has been
found out that in accordance with the present invention a
sufficient true surface area can be obtained even with a low
surface pressure when the surface smoothness of the current
collector is increased to a certain value or greater, and the
contact resistance becomes low.
[0042] The present invention can give a fuel cell that has a small
size, light weight, low cost, and high reliability at the same time
since a peripheral circuit can be mounted on the fuel cell main
body due to the use of a printed board as an end plate and,
furthermore, can employ a thin end plate since contact resistance
is low even at a low surface pressure due to the use of a current
collector having an appropriate surface configuration in a fuel
cell.
EXAMPLES
[0043] Preferred embodiments of the present invention are explained
below as Examples while comparing them with Comparative
Examples.
Example 1
[0044] A fuel cell having the structure shown in FIG. 2 was
actually prepared and the effects thereof were demonstrated. The
end plate shown in FIG. 2 was an FR-4 board having a square shape
with a long side of 66 mm and a thickness of 2 mm, and holes were
made by drilling. With regard to the drilled holes, eight holes
having a diameter of 2.2 mm were formed in the peripheral region
for assembly, and 56 holes having a diameter of 5 mm were formed as
through holes for outside air in a portion positioned on the
surface of a cathode electrode of an MEA.
[0045] An end plate with a circuit in FIG. 2 employed an FR-4
copper-clad laminate in which copper foil having a thickness of 100
.mu.m was affixed to an outer face of an irregular rectangular
shape having a length of 75 mm and a width of 66 mm and a thickness
of 2 mm, and a copper foil circuit with the patterning shown in
FIG. 3 was used.
[0046] As a fuel tank frame of FIG. 2, one formed by machining an
acrylic plate having a thickness of 3 mm into a frame shape was
used, and a polyethylene net having a thickness of 4 mm was
inserted into the frame.
[0047] With regard to the MEA, an electrolyte membrane was formed
by filling a polyethylene porous membrane with an electrolyte
polymer containing as main components
2-acrylamido-2-methylpropanesulfonic acid and
N,N'-methylenebisacrylamide, an oxygen electrode and a fuel
electrode were prepared by screen-printing carbon paper having an
area of 25 cm.sup.2 respectively with a reaction layer coating
solution containing a platinum-supporting carbon and a reaction
layer coating solution containing a platinum ruthenium
alloy-supporting carbon, and these were then hot-pressed to either
side of the electrolyte membrane to give an MEA.
[0048] A titanium expanded metal current collector having a
thickness of 0.1 mm was superimposed on either side of the MEA, a
polyethylene net having a thickness of 0.5 mm was further
superimposed on the expanded metal in order to adjust the surface
pressure, and the entirety was interposed between end plates and
tightened by means of eight M2.times.12 mm screws. At this stage,
the mean surface pressure of the entire contact surface was 1
kg/cm.sup.2. A titanium plate having a thickness of 0.05 mm and a
width of 8 mm was welded to the current collector, and fastened to
a printed circuit on the circuit-equipped end plate via a screw
(M2.times.12 mm) for tightening the main body as a lead for taking
electricity out of the cell.
[0049] The expanded metal was formed by subjecting a 0.1 mm thick
titanium plate to an expanding process and then a flattening
process in order to optimize irregularities on the surface to thus
squash the irregularities, and when the surface roughness was
measured using an atomic force microscope (AFM), Ra was 0.08 .mu.m
for a length of 20 .mu.m. The measurement is a mean value of
figures obtained with n=5 for the contact surface with the MEA
while avoiding end faces and obvious scratches.
[0050] The interior of the tank of the fuel cell thus formed was
filled with 6 mL of a 3 mol/L aqueous solution of methanol, and air
was used as an oxidizing agent. When the I-V characteristics were
measured, 1.2 V was obtained for 0.68 A, and a maximum output of
0.41 W per unit cell was obtained at 30.degree. C. Subsequently,
when the fuel cell was inserted into a socket of a commercial
portable electric appliance equipped with a socket for a printed
board, it could be made to operate well, and when fuel replacement,
etc. was required, the fuel cell main body was removed from the
socket, and maintenance could thus be carried out easily.
Comparative Example 1
[0051] A fuel cell was prepared in the same manner as in Example 1
except that the expanded metal for the current collector of Example
1 was not subjected to the flattening process although the
specifications for the material and the dimensions were the same,
and neither of the end plates at opposite sides was equipped with a
printed circuit. The lines of the net of the current collector that
had not been subjected to the flattening process made obliquely
twisted contact with the surface of the MEA, there was no flat
contact area, the roughness was too high, and measurement using AFM
could not be carried out. According to measurement was carried out
using a surface roughness meter, the roughness was 67.4 .mu.m for a
length of 4 mm with n=5.
[0052] When the maximum output per unit cell of this fuel cell was
measured by the same method as in Example 1, it was a very low
value of 0.13 W. It is surmised that this is due to a high contact
resistance between the current collector and the MEA. Furthermore,
since titanium, which was the material for the current collector
and the lead, could not be soldered, a circuit was formed by
compression-bonding a commercial insulated copper wire to the lead
with a compression bonding terminal and it was mounted on a
portable electric appliance that was the same as that in Example 1
except that it was not equipped with a socket, but mounting was
very difficult, when fuel was replaced the spent fuel needed to be
discharged by turning the portable electric appliance upside down,
and the operability was very poor.
INDUSTRIAL APPLICABILITY
[0053] The fuel cell of the present invention gives a low contact
resistance even at a low surface pressure by employing a current
collector having an appropriate surface configuration, it is
therefore possible to employ a thin end plate and, furthermore,
since a peripheral circuit can be mounted on a fuel cell main body
by using a printed board as the end plate, it is possible to
achieve small size, light weight, low cost, and high reliability at
the same time.
[0054] By exploiting the above-mentioned characteristics, this fuel
cell is very useful as a new power source as a replacement for an
existing dry battery or rechargeable battery, particularly in a
portable electric appliance.
* * * * *